The prior art includes numerous designs for temperature conditioning system controls that are directed to maintaining a zone at a comfortable temperature. The simplest control available for this purpose is a bi-metallic thermostat that is disposed within the comfort zone to energize one or more stages of temperature conditioning when the zone temperature deviates from a setpoint. Alternatively, the temperature conditioning capacity of a system may be varied in proportion to the difference between the zone temperature and the setpoint, as for example, by varying the speed of a refrigerant compressor used in heating or cooling the zone. Regardless of whether the temperature conditioning conditioning system includes one or more discrete stages, or its capacity is smoothly variable, this type control changes system capacity primarily in response to the comfort zone temperature. The temperature of the conditioned air supplied to the zone is generally allowed to rise or fall as required to satisfy the zone temperature conditioning demand.
There are also temperature conditioning systems in which the capacity of the system is controlled to provide a generally constant supply air temperature. The temperature within the zone is maintained at a setpoint using a flow regulator to adjust the volume of air conditioned to a substantially constant temperature, that is supplied to the zone. This type system is thus commonly referred to as a variable air volume (VAV) system, in contrast to the more conventional systems in which the flow of conditioned air is substantially constant.
Under certain conditions, both the constant volume and VAV systems can cause people in the comfort zone to experience uncomfortably cold drafts. For example, this may occur if the outdoor ambient temperature is relatively hot, resulting in a high demand for zone cooling. In attempting to maintain the comfort zone at the setpoint, a VAV system normally responds to the increased cooling demand by discharging 50.degree.-55.degree. F. supply air into the zone at very high airflow rates. A constant volume system likewise responds to an increased cooling demand by greatly increasing the capacity of the system causing very cold air, e.g., 35.degree.-40.degree. F., to be discharged into the zone. People directly exposed to the high velocity supply air in a VAV system or to the very cold supply air discharged from a constant volume system easily become chilled--even though a thermostat mounted in the zone away from the supply airstream discharge still has an unsatisfied cooling demand.
Conversely, in a heating mode, the minimum ventilation requirements for the zone may allow very cold ambient air from an outdoor air economizer to enter the supply airstream and cause discomfort to personnel in the zone. This is more likely to happen when the zone has a minimal heating demand and the percentage of cold outdoor air entering the zone is high due either to legal building code requirements for minimum ventilation or to the extensive use of high capacity exhaust fans, e.g., over restaurant grills or in paint booths.
In consideration thereof, it is an object of this invention to control a variable capacity temperature conditioning system to temper the air supplied to a comfort zone to prevent personnel in the zone being chilled by excessively cold supply air.
It is a further object to provide a supply air temperating control for a system operating in a heating and/or a cooling mode.
A still further object is to control the capacity of the temperature conditioning system to maintain the comfort zone at a setpoint, unless the supply air temperature drops below a predetermined minimum.
Yet a further object is to control the system to increase the supply air temperature if it should fall below the minimum, without causing the comfort zone temperature to rise more than a predetermined increment above the setpoint temperature.
These other objects of the invention will be apparent from the attached drawings and the description of the preferred embodiment that follows hereinbelow.